Sealed-off RF-excited gas lasers have evolved from early composite metal/ceramic CO2 waveguide devices to ceramic or all-metal structures. Examples of the early, waveguide devices are disclosed in U.S. Pat. No. 4,169,251, issued to Katherine D. Laakmann.
The basic "all-metal" laser structure, disclosed by Peter Laakmann in U.S. Pat. No. 4,805,182, represents the most recent configuration improvement for this class of laser. No combination of materials has been found that can improve on the cost-effectiveness of extruded aluminum as it is used in the "all-metal" laser. However, many remaining problems relating to sealed-off, RF-excited CO and CO2 discharge lasers still exist which are independent of the materials of construction. Challenges such as improving mode discrimination and power stability, increasing resonator efficiency, and increasing operating and shelf life apply to metal/ceramic, all-ceramic, and all-metal lasers. These problems can be addressed by improvements in optical configuration and improved processing methods.
A sealed-off gas laser that is configured neither as a waveguide laser nor as a free space laser shows the best efficiency and/or power output per unit length. True waveguide lasers with Fresnel numbers less than about 0.5 show good mode quality, easy adjustability and good electrical efficiency. However, their power output and operating life are limited because of optical losses in the waveguide cavity and because the optical power density is close to the damage threshold of mirror coatings. True free space optical configurations within transversely excited structures have the converse characteristics. They are sensitive to optical misalignment. Also, their electrical efficiency is low because the cross section of their plasma lasing medium must be larger than the optical mode diameter. Further, they operate at low optical power density and correspondingly low optical power extraction efficiency.
Lasers in the intermediate region (with diameter-to-length ratios of about 0.01, Fresnel numbers of about 1, and bore sizes in the range of 4-6 mm) offer performance that is superior to either waveguide or free space lasers. They rely on the walls to get a high optical fill factor. However, the interaction with the walls is not of a waveguiding nature and involves reflection of a small fraction of the optical beam power. Additionally, the plasma lasing medium has optical refractive power due to thermal gradients, and wall reflections can be used to minimize these optical losses. Very high single transverse mode electrical-to-optical conversion efficiencies, more than 20% for CO2 lasers, have been observed for these intermediate optical configurations. However, optical performance has not always been consistent and no explanation was available previously.
The shelf and operating lives of RF-excited gas lasers range up to several years and several hundred to several thousand hours, respectively. Shelf life is limited by the accumulation of water vapor due to hydrogen outgassing and oxidizing and to diffusion of water vapor through O-ring seals.
The accumulation of water vapor due to diffusion can be handled satisfactorily by either installing a water vapor getter (i.e., zeolite, cellulose), or by using hermetic seals. Combustion of hydrogen has been eliminated as a cause for limited operating life by most of the industry through the use of a high-temperature, high-vacuum bake-out process. Thus, oxygen loss is the principal factor that limits operating life.
Traditional methods to decrease oxygen loss during operation include various passivation methods. They involve burn-in of the finished laser and separate passivation of the internal surfaces. Most of these methods are very slow. For example, it may take a 48-hour burn-in to achieve an operating life of several hundred hours. This clearly limits the factory throughput. Faster methods are desirable.
A CO2 laser operates at temperatures that typically range from about 20 to 45 degrees Celsius. The oxygen species including ozone and atomic oxygen created during operation will slowly oxidize or be imbedded in the various internal surfaces of the laser's enclosure. Some of these processes are fast and some are slow, depending on oxygen species lifetimes and diffusion times. Many oxide species can be formed in the laser, given enough time, unless they already exist. If the oxide species do not already exist, the laser will eventually lose oxygen and its output power will diminish.
Internal laser parts made from ceramic and metal suffer from roughly the same oxidation or oxygen adsorption problem. Oxygen loss is also a problem for CO lasers since a very small amount of oxygen is needed to maintain discharge stability.
It is therefore desirable to have sealed-off RF-excited gas lasers that have improved mode discrimination and improved power stability, as well as increased operating and shelf life.